Recently, memory devices including non-volatile memory elements that store data by varying resistivity of variable resistors have been developed. Examples of such memory elements are a phase-change RAM (PRAM) that uses phase-change materials such as ternary chalcogenide, a colossal magnetoresistive effect (CMR) element that uses colossal magnetoresistance effects, and a resistive RAM (ReRAM) that uses variable resistance materials of transition metal oxide.
Writing data to the above enumerated memory elements is preformed by applying a voltage pulse of a predetermined condition to variable resistors in the memory elements, thereby varying resistance values of the variable resistors. Access control of the memory elements is performed via memory selection elements coupled to the memory elements.
Published Japanese Translation of PCT Application No. JP-T-2007-511895 discloses a conventional memory device that uses a transistor for a memory selection element. Published Japanese Translation of PCT Application No. JP-T-2005-522045 discloses a conventional memory device that uses a diode for a memory selection element. Japanese Patent Application Publication No. JP-A-2005-159359 discloses a conventional memory device that controls access to a memory element without using active elements such as a diode and a transistor.
Conventional semiconductor devices typically use variable resistive memory transistors or diodes for memory selection elements. Memory devices using transistors for memory selection elements are easy to control, and also high in reliability. Therefore, such memory devices are superior in data-holding performance and rewiring resistance. However, there is a disadvantage that, since a relatively large area is necessary for forming transistors, high integration of memory elements is difficult. Therefore, such memory devices are suitable for storing data, such as a boot program read during start-up and an OS, which require high reliability, but not suitable for storing data of large capacity.
Furthermore, memory devices using diodes for memory selection elements are suitable for increasing capacity since diodes can be formed in a smaller area compared to transistors and are also easier to stack vertically. However, access control to memory elements is performed by controlling a voltage level of bit lines and source lines coupled to the memory elements. Unfortunately, controlling the voltage can be difficult and unreliable. Accordingly, such memory devices are suitable for storing data with a large capacity such as moving image data or audio data, but unsuitable for storing important data on the other since their reliability is low and they are prone to cause problems such as disturbance and leakage. As such, in conventional memory devices, there is a problem that applicability thereof is limited depending on a characteristic of data to be stored.